Bridge plugs can be installed in pressurized wells, such as hydrocarbon well, to seal off a portion of the well, or to provide a barrier preventing flow of hydrocarbons from the well, or to prevent flow of gas, oil or water between zones down hole. Bridge plugs are usually set using an external setting tool to generate an axial force in the tool, resulting in radial expansion of an anchoring mechanism and a packer element towards the tubular inner wall. When the axial force provided by the setting tool reaches a predetermined level, the setting tool is disconnected from the plug and pulled out of the well, while the plug remains fixed to the casing or tubing down hole. The packer seals differential pressure above and below the tool and the anchoring mechanism prevents movement of the plug during the differential pressure period. A retrievable bridge plug is usually retrieved by lowering a pulling tool into the well. By mechanical manipulation, the pulling tool is latched to a fishing neck at the top of the tool, the plug is released from the tubing or casing, and pulled out of the well.
One problem in the use of such tools is the risk of retrievable plugs getting stuck when trying to release the plugs out of hole after having been used in a period for sealing differential pressure down hole. Problems with retrieving bridge plugs can include:
a) Sand or debris entering into vital parts in the release system and thereby preventing proper functioning of the mechanical. Sand, and/or debris can land on top of the plug as a result of fluid flow during equalizing carrying particles from the reservoir, perforation operations in the well above the plug, pressure testing causing scale and particles to fall down, fracturing operations with sand, or other operations that can cause settling of solids over the plug.
b) Deformation of elastomer in the plug such as swelling or permanent set caused by chemical and/or temperature effects on the material.
c) Explosive decompression in the elastomer element causing damage and/or deformation of the elastomer element.
d) Gap corrosion of sleeves in the tool that may over time prevent the proper function of release mechanisms, or sand building up in the annulus between a setting sleeve and the tubing or casing inner diameter.
Patents describing retrievable bridge plugs include U.S. Pat. No. 4,359,090, US2004/00244966, US2010/0186970, US2010/0019426, US2008/0060821, US2010/0288508, U.S. Pat. No. 7,290,603.
Equalizing systems, allowing the pressure above and below the tool to equalize exist in several known down hole tools in the form of ball valves sliding sleeves, shear screws, etc. One such system includes a long internal tube connecting an equalizing piston, below the packer to the fishing neck and operates by moving the fishing neck down.